Method of casting cylinders

ABSTRACT

A method for manufacturing aluminum cylinders wherein an aluminum or magnesium core including a plurality of protuberances is employed. The cylinder is die cast in a mold in which the core member is located. After removal from the mold, the entire main body portion of the cylindrical core is machined out leaving the protuberances remaining within the wall of the cast article. The article is mechanically impacted to loosen and knock out the protuberances from within the article.

United States Patent 1191 Frederickson 1 1 Oct. 29, 1974 METHOD OF CASTING CYLINDERS [75] Inventor: Robert Eugene Frederickson, South Gate, Calif.

[73] Assignee: McCulloch Corporation, Los

Angeles, Calif.

[22] Filed: Dec. 12, 1972 21 Appl. No.: 314,468

Related US. Application Data [63] Continuation-in-part of Ser. No. 20,202, March 17,

[52] US. Cl 164/132, 29/527.6, 164/345 [51] Int. Cl B22d 29/00 [58] Field of Search 164/131, 132, 138, 140;

[56] References Cited UNITED STATES PATENTS 4 1924 Pack 164/132 OTHER PUBLICATIONS Die Casting for Engineers, Copyright 1953, New Jersey Zinc Co., 160 Front St., New York 38, N.Y., Scientiflc Library TS253N4 1953C.2 page 12.

Primary Examiner-Andrew R. Juhasz Assistant Examiner-John S. Brown Attorney, Agent, or FirmBurns, Doane, Swecker & Mathis [5 7 ABSTRACT A method for manufacturing aluminum cylinders wherein an aluminum or magnesium core including a plurality of protuberances is employed. The cylinder is die cast in a mold in which the core member is located. After removal from the mold, the entire main body portion of the cylindrical core is machined out leaving the protuberances remaining within the wall of the cast articlefThe article is mechanically impacted to loosen and knock out the protuberances from within the article.

12 Claims, 9 Drawing Figures METHOD OF CASTING CYLINDERS RELATED APPLICATION This application is a continuation-in-part of US. Pat. application Ser. No. 20,202 filed Mar. 17, 1970 for Method of Casting Aluminum Cylinder."

BACKGROUND OF THE INVENTION This invention relates to the casting of hollow pieces by the use of what are usually characterized as permanent molds and which are hereinafter referred to merely as molds." More specifically, the invention is directed to an improved method and apparatus for die casting articles of manufacture having irregular hollow cavities and holes formed therein.

Manufacturers are currently faced with the problem of providing machinery, such as pumps and internal combustion engines, having higher requirements than have heretofore been called for with respect to cost, weight and tolerance criticality.

Pump bodies and internal combustion engine blocks are presently being manufactured by various die casting processes. A common method for casting lightweight internal combustion engine cylinders is to cast an aluminum body around an iron liner having various ports and blisters formed therein. Upon cooling, the aluminum cylinder having the iron liner cast therein, is machined to provide inlet and exhaust ports which extend entirely through the walls of the cylinder. This machining may be extensive and must be precise so as to constitute a time-consuming and expensive step in the cylinder forming process.

The iron liner about which the cylinder body is cast often has different heat expansion and heat transfer characteristics than the surrounding aluminum. As the result of these property differences, the iron liner and the surrounding body do not expand evenly when operationally incorporated in an internal combustion engine application. This uneven expansion causes distortion of the cylinder at elevated temperatures which distortion, in turn, causes a loss of engine power. Since distortion upsets the critical tolerances of an engine, the seal between piston rings and the inner bore surface of the cylinder becomes less effective and blow-by may result which has the effect of corroding various elements of the engine such as the wrist pins. Additionally, the different heat transfer coefficient of the iron liner with respect to the surrounding aluminum cylinder body presents cooling problems in conducting heat from within the cylinder, through the iron liner, through the aluminum body and, thence, to the cooling fins formed on the cylinder. Also, the existing need for lightweight engines, for example in chain saws, militates against the use of a heavy iron liner so that it would be an overall improvement in the art if many lightweight cylinder configurations could be easily formed without including such a liner.

The present invention is therefore directed to the die casting of articles such as aluminum internal combustion engine cylinders which heretofore have been traditionally constructed as composite articles.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a novel method for die casting hollow articles.

It is yet another object of the present invention to provide novel method for die casting internal combustion engine cylinders of a new, lightweight, unitary construction which satisfies close tolerance requirements.

The objects of the present invention are carried out by providing a method and an apparatus for casting hollow articles in which molten die casting material is introduced into a die having a core positioned therein. The die casting material is cooled until it solidifies and is then removed from the mold. A major portion of the core is machined from within the case article which is then mechanically impacted to knock-out the remaining portions of the core.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is particularly pointed out and distinctly claimed in the concluding portion of the specification and several embodiments are disclosed herein which may best be understood when taken in connection with the accompanying drawings, in which:

FIG. 1 is a sectional view taken along the axis of an internal combustion engine cylinder constructed in accordance with the method of the present invention;

FIG. 2 is a sectional view of the internal combustion engine cylinder of FIG. 1 taken along line 2-2 of FIG.

FIG. 3 is a pictorial view of a core member used to form the bore and various ports within the cast internal combustion engine cylinder shown in FIG. 1;

FIG. 4 is a pictorial view of the core member shown in FIG. 3 properly located within a mold for casting an internal combustion engine cylinder;

FIG. 5 is a schematic view showing blister forming protuberances on a core member which protuberances are formed with bosses extending radially through a cast cylinder wall into a wall of the die;

FIG. 6 shows the blister forming protuberances of FIG. 5 after the bosses have been mechanically impacted so as to knock the protuberances from within the cylindrical wall of the cast article;

FIG. 7 is a partial sectional view of a protuberance having a boss which is tapered to narrow toward the outer end thereof;

FIG. 8 is a partial sectional view of a protuberance being knocked out by a punch inserted within a hole formed in the cylinder wall; and

FIG. 9 is a partial sectional view of the portion of a cylinder shown in FIG. 8 after the protuberance has been knocked out and the hole properly sealed.

DETAILED DESCRIPTION Referring now to the drawings, in which like numerals are used to indicate like parts throughout the various views thereof:

FIG. I shows a sectional view of an internal combustion engine cylinder 10 having an internal bore 12. The cylinder 10 may be employed in the engine for a chain saw.

Acharge inlet port 14 is formed within the wall of the cylinder 10 near the lower portion thereof. An exhaust port 16 is formed within the wall of the cylinder 10 above the inlet port 14 so as to facilitate scavenging when the cylinder is used in connection with a two stroke internal combustion engine. By-pass or transfer ports 18 are formed as blisters within the wall of the cylinder 10. The ports 18 are positioned vertically between the inlet port 14 and exhaust port 16 and are spaced angularly therefrom. Each blister 18 may vary in configuration in accordance with the by-pass requirements of the specific engine design. Referring briefly to FIG. 2, it can be seen that the general extension of the blisters 18 along line e does not coincide with a radial r drawn to the approximate center of the blister. This offsetting angle, which may be approximately 30, is significant in that such angles are difficult to machine using current methods of manufacture.

Returning to FIG. 1, a generally cylindrical recess 20 is formed in the head of the cylinder which recess may be used to retain a spark plug (not shown). The generally spherical portion 22 defining the upper portion of the bore 12 may be used as the combustion chamber of the cylinder and may be of an irregular configuration. Cooling fins 24 are formed on the outer cylindrical surface of the cylinder 10 for effecting heat transfer from the combustion area 22 to the atmosphere surrounding the cylinder 10.

Referring now to FIG. 3, a core member 30 is shown which may be used to form the bore 12, the blisters 18, and the ports 14 and 16 of the internal combustion engine cylinder 10 shown in FIG. I. The core member 30 may consist of the same material as the engine cylinder 10 and may be formed with an inner cylindrical surface 32 which defines a recess for receiving a core arbor or positioner 50 of a die described hereinafter in reference to FIG. 4. The surface 32 may desirably be tapered. A radially projecting protuberance 38 is provided on the cylindrical surface of the core member 30 to form the exhaust port 16 shown in FIG. 1 and a second protuberance 40 forms the inlet port 14 shown in the same view. An irregular protuberance 42 is formed on the core member 30 vertically between the radial projections 38 and 40 but is angularly spaced therefrom to form the irregular blisters 18 shown in FIGS. 1 and 2. It will be noted that surface 44 of the projection 42 subtends an angle of approximately 30 with respect to a plane extending tangent to the cylindrical surface of the core 30 at a line common to the outer surface of the cylindrical core 30 and the surface 44 of the projection 42. The significance of this irregular configuration has been previously discussed with respect to FIGS. 1 and 2.

The core member 30 shown in FIG. 3 may be die cast of a lightweight metal such as aluminum or magnesium by any conventional means. If it is desired to die cast a hollow article such as pump body of a non-metallic substance or of any die castable material, it ispreferable, although not necessary, that the core member 30 comprise the same material as the die casting material to practice the present invention. In this connection, it may be noted that the provision of a core member 30 of magnesium may be advantageously employed in making a die cast hollow article of aluminum, without utilizing any anti-adhering substance of the type which is described more fully hereinafter.

Upon casting the core 30 of the same material as that comprising the hollow cast article, a heat and pressure resistant anti-adhering substance is coated at least on the protuberance surfaces of the core member 30 by a clipping, plating, brushing, or spraying operation.

The coating may comprise a substance such as a liquidized polyimide composition or a silicone oil. A coating of high temperature tetrafluoroethylene or hard chrome was found to be effective for the purposes of the present invention when applied to a core member 30 consisting of an aluminum alloy. The purpose of the coating is to facilitate the removal of the remaining portions of the core from the cylinder casting after machining the bore. Aluminum cores are currently being anodized with refractory materials in order to restrict the rate of heat transfer through the core, the cast article and the outer mold surfaces. It has been discovered, unexpectedly, that such refractory film is not satisfactorily effective to preclude fusion, welding or brazing of a cast aluminum article to an aluminum core member.

In the preferred embodiment of the present invention, a core member 30 consisting of aluminum or magnesium is cast and, if an anti-adhering substance is to be employed, the port forming blisters, ports and combustion chambers are sprayed, brushed or dipped with a liquidized polyimide composition which may be diluted with an appropriate solvent to comprise a solution comprising 50 percent to percent, by volume, of polyimide N-methylpyrrolidone has been found to be a suitable solvent for this purpose. The liquid film of the polyimide solution is then baked on the core 30 at 600 F. for 20 to 30 minutes. For best results, the core member 30 should be cleaned with the solvent and then dry heated before applying the polyimide composition. The cured coating should have a brown color and should not show any bare spots. The uniformity of the coating thickness is not critical.

As earlier noted, with a magnesium core 30, the coating step may be avoided.

The core member 30 is positioned (after coating, if the coating step is employed) within a mold as shown in FIG. 4 and properly held in place by means of the cylindrical locator portion 50 which projects upward from the bottom of the mold cavity 52 and, itself, provides the spherical surface 53 for forming the combustion chamber 22. The locator portion 50 may be tapered if the surface 32 of the core is tapered. Of course, the combustion chamber could alternatively be formed by a surface portion of the core member 30. The surrounding mold 54 may comprise any conventional mold member because the tolerances on the outside surfaces of a hollow article such as an engine cylinder are not generally critical. Therefore, if the material comprising the mold member 54 should expand at a different rate than the cast material, any resulting minor distortion would be acceptable.

After the core 30 is properly located within the mold 54, molten aluminum is introduced through an inlet duct 56 into the article forming surfaces of the mold member 54. The aluminum is first heated to a tempera ture within a range of 1,200 to 1,225 F. and enters the cavity 52 under a pressure within a range of 13,000 to 14,000 psi. The overall apparatus may then be cooled for 10 to 15 seconds until the aluminum has solidified at a temperature within a range of 600 to 700 F.

After the aluminum has solidified, the mold 54, which may be of the segmental type, is removed by any conventional means from around the article and the core 30 retained therein. The major portion of the core member 30 is then machined out from the cast article. The projections 38, 40 and 42 remain within the walls of the hollow cast article. The article is then mechanically impacted by means of a soft hammer, or the like, which impaction causes the remaining portions of the core member to be loosened from within the cast article and to fall out. An access hole may be drilled adjacent a blister forming projection to permit the knocking out of the projection by the insertion of a tool through the hole which tool may directly contact the projection and effect the removal thereof by transmitting mechanical impactions directly thereto.

ALTERNATIVE EMBODIMENTS FIG. 5 shows a core member 30 positioned within a die member 52 which core member 30 is formed with blister forming projections 42. The projections, in turn, are formed with radially extending tapered bosses 60 which extend through the cylinder and into holes 62 formed within the casting mold 52 for retaining the bosses 60.

FIG. 6 shows the cast cylinder after the mold 54 has been removed and after a major portion of the core member 30 has been machined out from therein. it will be appreciated, particularly with reference to FIG. 6, that the step of machining a major portion of the core from the cast article may involve boring out the generally cylindrical portion of the core member and leaving only the protuberances remaining distinctly encapsulated within the wall of the cast hollow article. The blister forming protuberances 42 have been displaced from the blisters within the cylinders by mechanically impacting the radial ends of the bosses 60 formed thereon. Each blister forming protuberance 42 is then broken off from the associated boss 60 at a neck portion 63 of the boss 60. The bosses 60 shown in FIGS. 5 and 6 are tapered to widen toward the radial ends thereof. This configuration permits the boss to plug the hole that was formed within the cylinder 10 thereby. The use of a boss 60 to apply a mechanical impaction directly to a blister forming projection 42 is especially useful when forming off-set transfer ports such as those shown in FIG. 2 which ports may extend at an angle which does not coincide with a radial line drawn from the center of the cylinder 10 to an approximate center of a blister 18. When the boss 60 is left within the cylinder wall to plug up the hole left thereby, it may be desirable to weld the outer portions of the boss 60 to the outer surface of the cylinder to prevent the boss from loosening and being propelled through the hole under the influence of any gas pressure buildup.

As shown in FlG. 7, the protuberance 42 may be formed with a boss 64 having a reverse tapered to that of boss 60 shown in FIG. 6. Alternatively, the protuberance 42 may be formed without a boss as shown in F lG. 8 and a hole 65 machined in the cylinder for the insertion of a punch tool 67 for removing the protuberance 42. A steel ball 69 and a sealing material'71 may then be used to fill the hole 65 as shown in FIG. 9.

If an aluminum core member 30 is utilized and it is permitted to form an oxide layer, before casting an aluminum article, it has been found that this layer may function as an anti-adhering layer to prevent fusion between the core and cast article. The formation of an aluminum oxide layer may be encouraged by placing a heated core in an atmosphere with an excess of oxygen.

The removal of core protuberances is facilitated if the protuberance is tapered toward the radial end thereof. It can be seen that the transfer port 18 of the cylinder shown in FIG. 1 has been formed by protuberance 42 of FIG. 3 which does so taper. If it should be that the protuberances left in the cylinder after the rough machining step are falling out too easily, the angle of taper may be reduced so that the cylinder retains a protuberance until mechanically impacted. The angle of protuberance taper is shown in the art as the draft." It will be appreciated that all the protuberances may have such draft.

It can thus be seen that an apparatus and method have been herein described for die casting irregular, hollow articles of manufacture such as internal combustion engine cylinders. The machining required after the molded material solidifies in the cast article is mainly rough machining of the interior of the cylinder which removes a major portion of the core member. Then, the article only need be impacted to remove the remaining portions of the core. The removed article may then be finely machined, plated and honed. Many of the advantages provided by the machining and impacting techniques of the present invention, are, of course realized independently of whether or not an anti-adhering substance is utilized.

Through this method and apparatus, it is no longer necessary to cast a lightweight aluminum cylinder around a heavy iron liner as is the present practice. Since there is no iron liner within the aluminum body of the cylinder, distortion of the critical tolerances of the overall cylinder is reduced when the engine is run at high operating temperatures. Likewise, since the cylinder is not a composite of dissimilar metals, cooling problems caused by having to conduct heat through such composites are avoided. Because the only machining required when practicing the present invention is the boring of a major portion of the core from the cylindrical body, the amount of expensive and timeconsuming machining is reduced. The closer tolerances now made available by the present invention provide a tighter piston fit, which in turn prevents compression loss and blowby which results in low engine power and the corrosion of engine parts such as wrist pins.

While what has been shown herein are several embodiments of the present invention, it is, of course, understood that the method and apparatus disclosed herein may be modified without departing from the invention. It is therefore intended to cover in the appended claims all such modifications as fall within the true scope and spirit of the present invention as defined in those claims:

What is claimed is: a

1. A method of molding hollow articles comprising the stepsof:

a. providing a mold having a cavity for forming the outer surface of the article;

b. providing a core member having an outer surface for forming the inner surface of the article, the outer surface of said core member including a main body portion and at least one protuberance projecting outwardly therefrom;

c. locating the core member within the cavity of the mold to define an article forming space between the outer surface of the core and the inner surface of the mold;

d. introducing molten die casting material into said article forming space and in surrounding relationship with said at least one protuberance,

e. cooling the molten material until it solidifies into a cast article with a configuration encapsulating said at least one protuberance;

f. releasing the solidified cast article from the mold;

g. machining the main body portion of the core from within the cast article leaving only said at least one protuberance remaining, to form a hollow article encapsulating said at least one protuberance of the core; and

h. mechanically impacting the cast article including the encapsulated said at least one protuberance of the core to loosen said encapsulated at least one protuberance free from the hollow article.

2. A method according to claim It wherein:

the step of providing a core member having an outer surface for forming the inner surface of the article comprises the step of casting a hollow generally cylindrical metal member forming said main body portion and having a plurality of said protuberances projecting therefrom for forming port holes and blisters within the hollow article;

the step of machining comprises forming a hollow article with each of said protuberances being distinctly encapsulated by said hollow article; and wherein the step of mechanically impacting is performed so as to loosen each of said distinctly encapsulated protuberances.

3. The method according to claim 2 wherein:

said core member is a magnesium core member.

4. The method according to claim 3 wherein:

said die casting material is aluminum.

5. The method according to claim 2 wherein the step of mechanically impacting comprises impacting the ends of bosses projecting from said protuberances, which bosses are left within the wall of said article to plug holes formed by said bosses.

6. The method according to claim 1 wherein the step of providing a core member comprises providing a core member with the main body portion of its outer article forming surface being generally cylindrical and having a plurality of said protuberances projecting outwardly therefrom for forming port holes and blisters within the hollow article.

7. A method of molding hollow articles comprising the steps of:

a. providing a mold having a cavity for forming the outer surface of the article;

b. providing a core member having an outer surface for forming the inner surface of the article, the outer surface of the core member being a generally cylindrical surface having protuberances projecting therefrom 'for forming port holes and blisters within the hollow article;

c. locating the core member within the mold;

d. introducing molten die casting material into the space defined by the outer surface of the core and the inner surface of the mold;

e. cooling the molten material until it solidifies;

f. releasing the solidified cast article from the mold;

g. machining a major portion of the core from within the cast article by the step of boring out the generally cylindrical portion of the core member and leaving only the protuberances remaining within the wall of the cast article;

h. mechanically impacting the article including the remaining portion of the core to loosen the remaining portion of the core free from the article;

comprises a metal selected from the group consisting of aluminum and magnesium.

9. A method of molding hollow articles comprising the steps of:

a. providing a mold having a cavity for forming the outer surface of the article;

b. providing a metal core member having an outer surface for forming the inner surface of the article, the outer surface of the core member being a generally cylindrical surface having protuberances projecting therefrom for forming port holes and blisters within the hollow article;

c. locating the core member within the mold;

d. introducing molten die casting material into the space defined by the outer surface of the core and the inner surface of the mold;

e. cooling the molten material until it solidifies;

f. releasing the solidified cast article from the mold;

g. machining a major portion of the core from within the cast article by the step of boring out the generally cylindrical portion of the core-member and leaving only the protuberances remaining within the wall of the cast article; and

h. mechanically impacting the article including the remaining portion of the core to loosen the remaining portion of the core free from the article.

10. A method for manufacturing hollow articles comprising the steps:

a. casting an article within a mold having a generally cylindrical core member located therein, the outer surface of the generally cylindrical core member including a main body portion and at least one protuberance projecting outwardly therefrom;

b. removing the article and core from the mold;

c. removing the entire main body portion of the generally cylindrical core from the article leaving only said at least one protuberance remaining within the wall of the cast article; and

d. mechanically impacting the article to loosen and knock out said at least one protuberance from within the article.

11. The method according to claim 10 wherein the step of mechanically impacting comprises impacting the end of a boss projecting from said at least one protuberance, which boss is left within the wall of said article.

12. A method for manufacturing hollow articles comprising the steps of:

a. casting an article within a mold having a core member located therein;

b. removing the article and core from the mold;

c. removing a major portion of the core from the article; and

d. mechanically impacting the article to loosen and knock out the remaining portion of the core from within the article;

e. the method including the additional steps of:

9 10 l. machining a hole through the hollow article and 4. transmitting a mechanical impaction through the extending to one of the remaining portions of the tool to the remaining portion of the core member core disposed therein; to perform said impacting step; and 2. inserting a punching tool into the hole; 5. sealing the hole after withdrawing the tool there- 3. positioning the tool in contact with the remain- 5 from.

ing portion of the core adjacent the hole;

1 UNETED- STATES PATENT UFFICE c E-iR; meATE F eesemg Patent No- 3, 844, 3134 Dated October 29, 1974 Inventor(s) Robert Eugene Frederickson It is certified that error appears in the above-identified patent and'that said Letters Patent are hereby corrected as shown below:

Column 6, line 4, after is change "shown" to known Signed and sealed this 18th day of February 1975,

(SEAL) Attes 1 C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks USCOMM-DC 60376-1 69 FORM PO-1050 (10-69) w u,s. GOVERNMENT PRINTING omcs I969 o-aes-sa4, 

1. A method of molding hollow articles comprising the steps of: a. providing a mold having a cavity for forming the outer surface of the article; b. providing a core member having an outer surface for forming the inner surface of the article, the outer surface of said core member including a main body portion and at least one protuberance projecting outwardly therefrom; c. locating the core member within the cavity of the mold to define an article forming space between the outer surface of the core and the inner surface of the mold; d. introducing molten die casting material into said article forming space and in surrounding relationship with said at least one protuberance; e. cooling the molten material until it solidifies into a cast article with a configuration encapsulating said at least one protuberance; f. releasing the solidified cast article from the mold; g. machining the main body portion of the core from within the cast article leaving only said at least one protuberance remaining, to form a hollow article encapsulating said at least one protuberance of the core; and h. mechanically impacting the cast article including the encapsulated said at least one protuberance of the core to loosen said encapsulated at least one protuberance free from the hollow article.
 2. A method according to claim 1 wherein: the step of providing a core member having an outer surface for forming the inner surface of the article comprises the step of casting a hollow generally cylindrical metal member forming said main body portion and having a plurality of said protuberances projecting therefrom for forming port holes and blisters within the hollow article; the step of machining comprises forming a hollow article with each of said protuberances being distinctly encapsulated by said hollow article; and wherein the step of mechanically impacting is performed so as to loosen each of said distinctly encapsulated protuberances.
 2. inserting a punching tool into the hole;
 3. The method according to claim 2 wherein: said core member is a magnesium core member.
 3. positioning the tool in contact with the remaining portion of the core adjacent the hole;
 4. The method according to claim 3 wherein: said die casting material is aluminum.
 4. transmitting a mechanical impaction through the tool to the remaining portion of the core member to perform said impacting step; and
 5. sealing the hole after withdrawing the tool therefrom.
 5. The method according to claim 2 wherein the step of mechanically impacting comprises impacting the ends of bosses projecting from said protuberances, which bosses are left within the wall of said article to plug holes formed by said bosses.
 6. The method according to claim 1 wherein the step of providing a core member comprises providing a core member with the main body portion of its outer article forming surface being generally cylindrical and having a plurality of said protuberances projecting outwardly therefrom for forming port holes and blisters within the hollow article.
 7. A method of molding hollow articles comprising the steps of: a. providing a mold having a cavity for forming the outer surface of the article; b. providing a core member having an outer surface for forming the inner surface of the article, the outer surface of the core member being a generally cylindrical surface having protuberances projecting therefrom for forming port holes and blisters within the hollow article; c. locating the core member within the mold; d. introducing molten die casting material into the space defined by the outer surface of the core and the inner surface of the mold; e. cooling the molten material until it solidifies; f. releasing the solidified cast article from the mold; g. machining a major portion of thE core from within the cast article by the step of boring out the generally cylindrical portion of the core member and leaving only the protuberances remaining within the wall of the cast article; h. mechanically impacting the article including the remaining portion of the core to loosen the remaining portion of the core free from the article; i. the step of introducing molten material into the space defined between the core and mold comprising the step of supplying molten aluminum heated within the range of 1,200*-1, 225* F. and subjected to a pressure within the range of 13,000 - 14,000 psi into the space.
 8. The method of claim 5 wherein the core member comprises a metal selected from the group consisting of aluminum and magnesium.
 9. A method of molding hollow articles comprising the steps of: a. providing a mold having a cavity for forming the outer surface of the article; b. providing a metal core member having an outer surface for forming the inner surface of the article, the outer surface of the core member being a generally cylindrical surface having protuberances projecting therefrom for forming port holes and blisters within the hollow article; c. locating the core member within the mold; d. introducing molten die casting material into the space defined by the outer surface of the core and the inner surface of the mold; e. cooling the molten material until it solidifies; f. releasing the solidified cast article from the mold; g. machining a major portion of the core from within the cast article by the step of boring out the generally cylindrical portion of the core member and leaving only the protuberances remaining within the wall of the cast article; and h. mechanically impacting the article including the remaining portion of the core to loosen the remaining portion of the core free from the article.
 10. A method for manufacturing hollow articles comprising the steps: a. casting an article within a mold having a generally cylindrical core member located therein, the outer surface of the generally cylindrical core member including a main body portion and at least one protuberance projecting outwardly therefrom; b. removing the article and core from the mold; c. removing the entire main body portion of the generally cylindrical core from the article leaving only said at least one protuberance remaining within the wall of the cast article; and d. mechanically impacting the article to loosen and knock out said at least one protuberance from within the article.
 11. The method according to claim 10 wherein the step of mechanically impacting comprises impacting the end of a boss projecting from said at least one protuberance, which boss is left within the wall of said article.
 12. A method for manufacturing hollow articles comprising the steps of: a. casting an article within a mold having a core member located therein; b. removing the article and core from the mold; c. removing a major portion of the core from the article; and d. mechanically impacting the article to loosen and knock out the remaining portion of the core from within the article; e. the method including the additional steps of: 